In medical terminology, an addiction is a chronic neurobiologic disorder that has genetic, psychosocial, and environmental dimensions and is characterized by one of the following: the continued use of a substance despite its detrimental effects, impaired control over the use of a drug (compulsive behavior), and preocupation with a drug's use for non-therapeutic purposes (i.e. craving the drug). Addiction is often accompanied the presence of deviant behaviors (for instance stealing money and forging prescriptions) that are used to obtain a drug.

Tolerance to a drug and physical dependence are not defining characteristics of addiction, although they typically accompany addiction to certain drugs. Tolerance is a pharmacologic phenomenon where the dose of a medication needs to be continually increase in order to maintain its desired effects. For instance, individuals with severe chronic pain taking opiate medications (like morphine) will need to continually increase the dose in order to maintain the drug's analgesic (pain-relieving) effects. Physical dependence is also a pharmacologic property and means that if a certain drug is abruptly discontinued, an individual will experience certain characteristic withdrawal signs and symptoms. Many drugs used for therapeutic purposes produce withdrawal symptoms when abruptly stopped, for instance oral steroids, certain antidepressants, benzodiazepines, and opiates.

However, common usage of the term addiction has spread to include psychological dependence. In this context, the term is used in drug addiction and substance abuse problems, but also refers to behaviors that are not generally recognized by the medical community as problems of addiction, such as compulsive overeating.

The term addiction is also sometimes applied to compulsions that are not substance-related, such as problem gambling and computer addiction. In these kinds of common usages, the term addiction is used to describe a recurring compulsion by an individual to engage in some specific activity, despite harmful consequences, as deemed by the user himself to his or her individual health, mental state or social life.